A key measure of the performance of an electromagnetic information storage system is the areal density. The areal density is the number of data bits that can be stored and retrieved in a given area. Areal density can be computed as the product of linear density (the number of magnetic flux reversals or bits per unit distance along a data track) multiplied by the track density (the number of data tracks per unit distance). As with many other measures of electronic performance, areal densities of various information storage systems have increased greatly in recent years. For example, commercially available hard disk drive systems have enjoyed a roughly tenfold increase in areal density over the last few years, from about 500 Mbit/in2 to about 5 Gbit/in2.
Various means for increasing areal density are known. For instance, with magnetic information storage systems it is known that storage density and signal resolution can be increased by reducing the separation between a transducer and associated media. For many years, devices incorporating flexible media, such as floppy disk or tape drives, have employed a head in contact with the flexible media during operation in order to reduce the head-media spacing. Recently, hard disk drives have been designed which can operate with high-speed contact between the hard disk surface and the head.
Another means for increasing signal resolution that has become increasingly common is the use of magnetoresistive (MR) or other sensors for a head. MR elements may be used along with inductive writing elements, or may be separately employed as sensors. MR sensors may offer greater sensitivity than inductive transducers but may be more prone to damage from high-speed contact with a hard disk surface, and may also suffer from corrosion, so that conventional MR sensors are protected by a hard overcoat.
Recent development of information storage systems having heads disposed within a microinch (μin) of a rapidly spinning rigid disk while employing advanced MR sensors such as spin-valve sensors have provided much of the improvement in areal density mentioned above. Further increases in linear density have been hampered by demagnetizing forces from adjacent bits, which grow stronger as the bits are packed closer together, typically manifested as nonlinear transition shifts (NLTS) of longitudinal media. On the other hand, further increases in track density have been limited by constraints as to how small transducer pole-tips can be made, since the pole-tips record magnetic patterns on the media and therefore define the width of each track.
FIG. 1 (Prior Art) depicts a portion of a conventional thin film head 50 as seen from a media on which the head writes and reads. The head contains a transducer formed in a series of layers on a substrate 51, the transducer including a MR sensor 52 sandwiched between a pair of magnetically permeable shield layers 54 and 55. Layer 55 also serves as a first pole-tip of a magnetically permeable yoke that encircles a conductive coil, not shown, the first pole-tip 55 being separated from a second pole-tip 58 by a recording gap 60. In this example of a merged MR and inductive head, reading of signals is performed by the MR sensor 52, while writing of patterns on the media is performed by magnetic flux spreading out from the gap 60 while travelling between the pole-tips 55 and 58. A width W0 of the trailing pole-tip 58 thus sets a minimum width of a data track recorded on the medium. Conventional pole-tips 55 and 58 are formed by sputtering a seed layer followed by patterning a photoresist mask for electroplating, to form a layer that may be a few microns thick for carrying sufficient magnetic flux to provide adequate recording strength to the media. After electroplating through the thick mask, the mask is chemically removed and the seed layer is removed by ion beam milling, which can also be used to thin the pole-tip.
Control of the ion milling for thinning pole-tips becomes difficult for widths W0 that are less than 0.5 μm, and errors in mask definition increase with mask thickness, limiting a length-to-width aspect ratio of conventional pole-tips to less than six. Instead of ion milling at the wafer level, trimming of a pole-tip with a focused ion beam impinging upon the air-bearing surface has been proposed. Unfortunately, this leaves a cavity in that surface around the pole-tip, and tends to round the corners of the pole-tip adjacent the cavity. Moreover, trimming with an individual beam for each pole-tip is not competitive with mass production of pole-tips at the wafer level.